The invention relates to a brush wear detector for use on a dynamoelectric machine and in combination with a wear indicating circuit. More particularly, the invention relates to a detector that when mounted in operating relationship on such a machine, is responsive to a predetermined degree of movement of a brush-biasing spring to close an electrical switch contactor and so cause the energization of a brush wear indicating signal circuit.
The use of brush wear detectors and associated brush wear indicating circuits to alert operators of electric machines to the occurrence of a given degree of wear of brushes on the machines is generally well known. In applications of such machines where unscheduled outages are particularly undesirable, the extra costs associated with providing a brush wear indicating function for the machines is commercially acceptable. For example, in certain motors and generators used on vehicles that are often operated for extended periods in locations remote from garages or other convenient maintenance or repair facilities, the importance of warning operating or maintenance personnel to replace extensively worn brushes, in an appropriately timely manner, often mandates the use of brush wear indicators on the machines. The operation of brush wear detectors in such important applications makes it desirable that they be made to reliably respond to the occurrence of a predetermined degree of brush wear. Moreover, the detectors should be capable of energizing an indicating circuit at an accurately set point corresponding to a precisely predetermined degree of wear of an associated brush.
In addition to such operating objectives, brush wear detectors must be able to perform effectively in the relatively harsh environments frequently encountered in their application. Considerable mechanical stress is usually transmitted to brush wear detectors due to both the shocks and vibrations to which an entire motor or generator on which a brush wear detector is mounted, and due to extensive variations in the electric current passing through the windings, communtator and brushes of the motor or generator on which the detector is applied. Finally, the common presence of contaminants, such as carbon dust and other particulate matter from the ambient, all combine to impose stringent requirements on designers of brush wear detectors.
A relatively early type of brush wear detector and indicator arrangement is shown in U.S. Pat. No. 2,691,114, which issued Oct. 5, 1954. The mechanism disclosed in that patent uses an electrical contactor mounted on a brush-follower spring to close a signal circuit responsive to a brush being worn sufficiently to enable the contactor-carrying spring arm to move close enough to a fixed contactor in the path of movement of the spring arm to effect such a circuit completion. One disadvantage of such an arrangement is that the engagement of the movable contactor with the fixed contactor can be easily interrupted due to vibration of the associated brush, thereby causing the brush-follower arm to bounce and intermittently disengage the movable contactor from the fixed contactor. Such tentative contact closure is avoided by other types of brush wear indicators, one example of which is shown in U.S. Pat. No. 3,523,288, that issued Aug. 4, 1970. With the device disclosed in that patent positive switch opening or closing action is obtained for a brush wear indicator by providing an insulated pin that slides on one side of an associated brush until a recess that is machined into the brush at a predetermined point near its upper end is moved under the pin. When such a range of movement occurs, the pin drops into the recess, thus causing a switch in the indicator circuit to snap either open or closed, thereby providing a signal to an associated wear indicating circuit. One disadvantage of such an arrangement is that the use of such a pin as a switch actuator produces a constant drag on the associated carbon brush. Moreover, in that type of arrangement, the pin actually acts to arrest movement of the carbon brush once the pin has dropped into the recess in the brush and into contact with one wall thereof.
The problem of undesirable drag on movement of a brush is avoided by another type of previously known brush wear indicator arrangement, as shown in U.S. Pat. No. 4,024,525, which issued May 17, 1977. In that patent, a brush wear detector probe is mounted on an insulating member adjacent to an elongated groove formed in the side wall of an associated brush. As the brush wears down during its normal operation, the probe is positioned within the groove and spaced from the walls thereof so that it does not conduct current from the brush through the probe to an associated brush wear indicating circuit. When due to wear of the brush, it has moved down sufficiently to bring the end of the probe into contact with the uppermost wall of the groove in the brush, current is conducted from the brush through the probe to the indicator circuit. Although the probe does not restrict brush movement during most of the wear cycle of the brush, the probe does arrest further downward movement of the brush when the probe finally engages the upper wall of the groove. Reduced or intermittant current through the brush, resulting from such a stoppage of brush movement, can limit or impair operation of the associated dynamoelectric machine, until a new brush is installed.
A later U.S. Pat. No. 4,172,988, issued Oct. 30, 1979, discloses a brush wear indicating means that may partially alleviate the problem of downward movement of a brush being arrested by engagement thereof by a brush wear detector mechanism. In that patent, a movable contact is mounted on a relatively long lever arm and is driven into engagement with a fixed contact mounted on an insulating member positioned adjacent to the path of movement of the brush, as a brush-follower spring forces the brush and the movable contact downward. Due to the long lever arm mounting the movable contactor, it is possible for the brush to be pivoted slightly around the fulcrum point of that arm to enable further wear of the brush after the wear-indicating contactors have been engaged. However, the disclosed mechanism still has the inherent disadvantage of severely restricting movement of the brush after the contactors are engaged. Also, this disclosed mechanism is somewhat like the brush wear indicator disclosed in the first patent mentioned above, in that movement of a wear indicating contactor is achieved by using the direct driving thrust of a brush-follower spring to move the contactor. Accordingly, intermittent and weak wear-indicating signals may result due to vibrations being transmitted through the brush to the juxtaposed surfaces of the movable and fixed signal circuit contactors when a predetermined degree of brush wear has been achieved.
It has been recognized that a brush-follower spring of the pre-stressed convoluted type now commonly found in commercial brushholder applications can be used to actuate a brush wear indicating circuit in a way that minimizes the undesirable transmission of vibration from an associated brush to the movable switch contactor of a brush wear indicating circuit. In that regard, U.S. Pat. No. 4,272,695, which issued June 9, 1981 discloses a brush wear indicator mechanism in which a movable contactor-bearing leaf spring is mounted on a support structure to position a movable end of the leaf spring against an uncoiled portion of a brush-biasing spring that has its coiled portion positioned on the upper end of a brush to drive it against an associated commutator or slip ring. As the brush wears, the brush-biasing spring winds up and causes the uncoiled portion to move in a direction away from the free end of the leaf spring. Such movement enables the leaf spring to drive the contactor mounted on it into engagement with a fixed contactor that is mounted on a suitably insulated terminal in the path of movement of the movable contactor. Closure of the two contactors is effective to initiate transmission of a brush wear indicating signal to an associated indicating circuit. A disadvantage of the type of apparatus shown in this patent is that the leaf spring bears against the brush-biasing spring and is energized by it. That arrangement can cause the leaf spring to significantly interfere with the driving force of the brush-biasing spring. In addition, such an arrangement results in the movable contactor being energized at all times, so it is necessary to provide a separately insulated fixed contactor on the brushholder assembly to which the brush wear indicating circuit is connected. It would be desirable to eliminate the added expense inherent in providing such an additional insulated contactor.
Finally, in an earlier filed U.S. patent application, Ser. No. 183,904, which was filed Sept. 4, 1980 and issued May 11, 1982 as U.S. Pat. No. 4,329,683 which is assigned to the assignee of the present invention, there are disclosed several embodiments of brush wear indicators that, respectively, utilize various electro-mechanical transducers operably connected between the coiled portion of a brush-biasing spring and a brush wear signal means to actuate a signal in response to the coiled portion of the spring being moved a predetermined distance. In each of those mechanisms, the transducer is directly connected in line with the driving thrust of the coiled portion of the spring, as it responds directly to brush movement in its control of a movable contactor of a wear-indicating circuit. Means are provided in each of those devices to reduce friction interference between the transducers and the brush-biasing spring. However, due to the direct in-line engagement of the transducers with the respective coiled portions of associated brush-biasing springs, the devices disclosed in that patent do have the disadvantage of transmitting an undesirable amount of vibration from the brush to the wear-indicating circuit contactor. Moreover, the contactor mounting means and spring actuating means disclosed in that patent are relatively expensive to manufacture, complex in structure and space consuming to assemble. Frequently in the context of conventional brushholder assemblies, there is a very limited amount of space available for installing desired brush wear indicators and associated brush wear detector mechanisms, therefore, it is desirable to provide a brush wear detector that requires relatively little space for its installation and operation on a brushholder assembly.